Roman Abramovich gave Vladimir Putin a $35 million (?25m) yacht in one of dozens of acquisitions through which the Russian president has amassed a vast covert fortune, it has been claimed.
The remarkable allegations come as a top American official openly accused Mr Putin of corruption, nepotism, and direct embezzlement of state funds.
While Mr Putin?s personal wealth has been the subject of speculation and lurid claims of corruption for years, little evidence has ever been produced to back up such allegations.
Mr Putin himself has dismissed claims about his wealth as ?simply rubbish.?
But a new Panorama documentary to be aired on BBC One tonight details allegations that Mr Putin has established a complex system of ?training and practices in term of how to mask his actual wealth.?
Details include Mr Putin?s alleged acquisition of the Olympia, a 57-metre luxury yacht.
Dmitry Skarga, a former head of Russia?s state shipping company Sovcomflot, said the boat was a gift from Mr Abramovich, who transferred it to the president via an intermediary.
?It's a fact that Mr Abramovich, through his employee transferred a yacht to Mr Putin,? Mr Skarga told Panorama. Mr Skarga also alleged that the yacht is run and maintained with state funds, even though it is Mr Putin?s personal property.
Adam Szubin, an official who oversees sanctions at the US Treasury, openly told the program that he believes Mr Putin is ?corrupt,? and that the United States has been aware of Mr Putin?s graft for ?many, many years.?
?We've seen him enriching his friends, his close allies, and marginalising those who he doesn't view as friends using state assets. Whether that's Russia's energy wealth, whether it's other state contracts, he directs those to whom he believes will serve him and excludes those who don't. To me, that is a picture of corruption,? Mr Szubin said in comments to be broadcast on Panorama tonight.
While Mr Szubin did not put a figure on the size of Mr Putin?s fortune, he did say that the Russian president?s declared annual salary is ?not an accurate statement of the man?s wealth.?
In 2007 Stanislav Belkovsky, a Russian political commentator, claimed Mr Putin had amassed a $40 billion fortune through stakes in three Russian energy companies.
Last year Bill Browder, a hedge fund manager and vocal critic of Mr Putin, claimed that the Russian president is worth $200 billion - a sum that would make him by far the richest man in the world.
In 2014 the US government stated that Mr Putin had secret investments in the energy sector, but did not put a value for them and did not openly accuse him of corruption.
Little evidence has ever been produced to back up those claims, and Mr Putin?s spokesman has called them ?complete trash.?
The Kremlin told the BBC that the allegations did not require a response ?as they are pure fiction.?
Mr Abramovich?s lawyers called the claims about the yacht ?unsubstantiated? and little more than a ?rehash of speculation and rumours?.